Using two-way exchange between coherent frequency combs, each phase-locked to the local optical oscillator, optical time–frequency transfer is demonstrated in free space across a 2-km-long link, with a timing deviation of 1 fs, a residual instability below 10−18 at 1,000 s and systematic offsets below 4 × 10−19.
- Fabrizio R. Giorgetta
- William C. Swann
- Nathan R. Newbury
